Hot-rolled steel sheet, method of manufacturing the same, and equipment for manufacturing the same

ABSTRACT

A method of manufacturing a hot-rolled steel sheet is explained. The method includes hot-rolling a slab, performing a first phase transformation process cooling a surface of the hot-rolled slab to phase-transform a structure of a surface portion of the hot-rolled slab, performing a second phase transformation process to phase-transform a structure of a central portion of the hot-rolled slab after the first phase transformation process, and coiling the slab including the phase-transformed surface portion and the phase-transformed central portion.

CROSS-REFERENCE TO RELATED APPLICATION

This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2014-0000260, filed on Jan. 2, 2014, in the Korean Intellectual Property Office, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND

The inventive concepts relate to a hot-rolled steel sheet, a method of manufacturing the same, and equipment for manufacturing the same. More particularly, the inventive concepts relate to a hot-rolled steel sheet manufactured by performing a first phase transformation process of cooling a surface portion of a hot-rolled slab and a second phase transformation process of cooling a central portion of the slab, a method of manufacturing the same, and equipment for manufacturing the same.

A hot-rolled steel sheet means a steel sheet formed by rolling a board-shaped slab at a high temperature. Hot-rolled steel sheets are widely used in various industrial fields such as wallboards of a building, a steel pipe for transporting oil, a steel sheet for welding, a steel sheet for a ship, and a steel sheet for a car.

Formability of the hot-rolled steel sheets may be improved in order to use the hot-rolled steel sheets in the various industrial fields. In order to improve the formability of the hot-rolled steel sheets, various researches are carried out to improve the elongation of the hot-rolled steel sheets and to increase the strength of the hot-rolled steel sheets. However, as the strength of the steel sheets increases, the formability of steel sheets may be generally deteriorated. Thus, it may be difficult to use steel sheets having high strength as car parts.

To resolve this problem, Korean Laid Open Patent Application No. 2000-0043784 (Korean patent application No. 10-1998-0060205, Applicant: POSCO) suggests a high strength hot-rolled steel sheet composed of carbon of 0.06 wt % to 0.1 wt %, silicon of 0.3 wt % or less, manganese of 1.4 wt % to 2.0 wt %, phosphorus of 0.02 wt % or less, sulfur of 0.005 wt % or less, aluminum of 0.01 wt % to 0.05 wt %, titanium of 0.05 wt % to 0.15 wt %, niobium of 0.02 wt % to 0.04 wt %, nitrogen of 50 ppm or less, a residual iron (Fe), and other inevitable impurities and a method of manufacturing the same. This high strength hot-rolled steel sheet has a tensile strength of about 70 kg/mm²

Korean Laid Open Patent Application No. 2001-0060647 (Korean patent application No. 10-1999-0063053, Applicant: POSCO) suggests a hot-rolled steel sheet composed of carbon of 0.06 wt % to 0.10 wt %, silicon of 0.5 wt % to 1.0 wt %, manganese of 1.5 wt % to 2.0 wt %, phosphorus of 0.02 wt % or less, sulfur of 0.0005 wt % or less, aluminum of 0.010 wt % to 0.050 wt %, titanium of 0.050 wt % to 0.10 wt %, niobium of 0.020 wt % to 0.040 wt %, nitrogen of 60 ppm or less, a residual iron (Fe), and other inevitable impurities and a method of manufacturing the same. This hot-rolled steel sheet has an excellent manufacturing property and a tensile strength of about 780 MPa.

SUMMARY

Embodiments of the inventive concepts may provide a high strength hot-rolled steel sheet, a method of manufacturing the same, and equipment for manufacturing the same.

Embodiments of the inventive concepts may also provide a high elongation hot-rolled steel sheet, a method of manufacturing the same, and equipment for manufacturing the same.

Embodiments of the inventive concepts may further provide a method of manufacturing a hot-rolled steel sheet capable of easily being applied to general processes, and equipment for manufacturing the same.

Embodiments of the inventive concepts may further provide a car part including a high strength and high elongation hot-rolled steel sheet.

In one aspect, a method of manufacturing a hot-rolled steel sheet may include: hot-rolling a slab; performing a first phase transformation process cooling a surface of the hot-rolled slab to phase-transform a structure of a surface portion of the hot-rolled slab; performing a second phase transformation process to phase-transform a structure of a central portion of the hot-rolled slab after the first phase transformation process; and coiling the slab including the phase-transformed surface portion and the phase-transformed central portion.

In some embodiments, a phase of the structure of the central portion of the hot-rolled slab may be maintained during the first phase transformation process.

In some embodiments, a phase of the structure of the surface portion of the hot-rolled slab may be maintained during the second phase transformation process.

In some embodiments, performing the first phase transformation process may include: providing compressed air to the surface portion of the hot-rolled slab.

In some embodiments, performing the first phase transformation process may include: jetting a liquid temperature-reducing material to the surface portion of the hot-rolled slab.

In some embodiments, the hot-rolled slab may have an austenite structure. In this case, the surface portion of the hot-rolled slab may have a ferrite structure by the first phase transformation process, and the central portion of the hot-rolled slab may have the austenite structure after the first phase transformation process and before the second phase transformation process. The central portion of the hot-rolled slab may have a structure having a higher strength than the ferrite structure by the second phase transformation process, and the surface portion of the hot-rolled slab may have the ferrite structure after the second phase transformation process.

In some embodiments, an elongation of the surface portion phase-transformed by the first phase transformation process may be higher than an elongation of the central portion phase-transformed by the second phase transformation process. A strength of the central portion phase-transformed by the second phase transformation process may be higher than a strength of the surface portion phase-transformed by the first phase transformation process.

In some embodiments, performing the second phase transformation process may include: water-cooling the hot-rolled slab.

In some embodiments, the method may further include: tempering the coiled slab.

In some embodiments, the first phase transformation process and the second phase transformation process may be performed using different apparatuses from each other.

In another aspect, a hot-rolled steel sheet may include: a first surface; a second surface opposite to the first surface; and a central portion between the first and second surfaces. A first phase may have a maximum volume fraction and a second phase may have a minimum volume fraction at the first surface and the second surface. The first phase may have a minimum volume fraction and the second phase may have a maximum volume fraction in the central portion.

In some embodiments, a structure of the first phase may be a ferrite structure, and a structure of the second phase may have a higher strength than the ferrite structure.

In some embodiments, a structure of the first phase may have a higher elongation than a structure of the second phase, and the structure of the second phase may have a higher strength than the structure of the first phase.

In some embodiments, first and second portions respectively adjacent to the first and second surfaces may have only the first phase, and the central portion may have only the second phase.

In some embodiments, the volume fraction of the first phase may progressively decrease from the first surface to the central portion, and the volume fraction of the second phase may progressively increase from the first surface to the central portion. The volume fraction of the first phase may progressively increase from the central portion to the second surface, and the volume fraction of the second phase may progressively decrease from the central portion to the second surface.

In still another aspect, a hot-rolled steel sheet may include: a first portion having only a first phase; a first mixed portion disposed on the first portion, the first mixed portion having the first phase and a second phase different from the first phase; a central portion disposed on the first mixed portion, the central portion having only the second phase; a second mixed portion disposed on the central portion, the second mixed portion having the first phase and the second phase; and a second portion disposed on the second mixed portion, the second portion having only the second phase.

In some embodiments, a structure of the first phase may be a ferrite structure, and a structure of the second phase may have a higher strength than the ferrite structure.

In some embodiments, a structure of the first phase may have a higher elongation than a structure of the second phase, and the structure of the second phase may have a higher strength than the structure of the first phase.

In some embodiments, a volume fraction of the first phase may be higher than a volume fraction of the second phase in a region of the first mixed portion adjacent to the first portion and in a region of the second mixed portion adjacent to the second portion. A volume fraction of the second phase may be higher than a volume fraction of the first phase in another region of the first mixed portion adjacent to the central portion and in another region of the second mixed portion adjacent to the central portion.

In still another aspect, a car part manufactured using the hot-rolled steel sheet is provided.

In still another aspect, equipment for manufacturing a hot-rolled steel sheet may include: a roller hot-rolling a slab; a transport table transporting the hot-rolled slab; a first surface cooling part disposed over the transport table, the first surface cooling part cooling a first surface of the hot-rolled slab; a second surface cooling part spaced apart from the first surface cooling part with the transport table therebetween, the second surface cooling part cooling a second surface of the hot-rolled slab opposite to the first surface; and a third cooling part water-cooling the slab cooled by the first and second surface cooling parts.

In some embodiments, the transport table may transport the slab in a first direction. Each of the first and second surface cooling parts may include: a temperature-reducing material supply line extending in a second direction perpendicular to the first direction; and a temperature-reducing material supply nozzle supplying a temperature-reducing material supplied from the temperature-reducing material supply line to the first surface or the second surface of the hot-rolled slab.

In some embodiments, the temperature-reducing material supply line may be provided in plural in each of the first and second surface cooling parts, and the plurality of temperature-reducing material supply lines may be spaced apart from each other and arranged in the first direction.

In some embodiments, the temperature-reducing material supply nozzle may include a plurality of temperature-reducing material supply nozzles spaced apart from each other in the second direction and installed on the temperature-reducing material supply line.

In some embodiments, the temperature-reducing material supply nozzle may supply the temperature-reducing material to the first surface or the second surface of the slab in an opposite direction to the first direction.

In some embodiments, the temperature-reducing material supply nozzle may be disposed to be oblique with respect to the first surface or the second surface of the slab.

In some embodiments, the temperature-reducing material may include compressed air.

In some embodiments, the temperature-reducing material may include a liquid temperature-reducing material, and the liquid temperature-reducing material may be jetted from the temperature-reducing material supply nozzle.

In some embodiments, a first portion of the slab adjacent to the first surface and a second portion of the slab adjacent to the second surface may be phase-transformed by the first and second surface cooling parts, and a phased of a central portion of the slab between the first and second portions may be maintained when the first and second portions are phase-transformed.

In some embodiments, the central portion of the slab may be phase-transformed by water supplied from the third cooling part, and the phases of the first and second portions adjacent to the first and second surfaces may be maintained when the central portion is phase-transformed.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concepts will become more apparent in view of the attached drawings and accompanying detailed description.

FIG. 1 is a flowchart illustrating a method of manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 2 is a perspective view illustrating a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 3 is a cross-sectional view taken along a line A-A′ of FIG. 2 to illustrate a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 4 is an enlarged cross-sectional view taken along a line A-A′ of FIG. 2 to illustrate a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 5 is a perspective view illustrating equipment for manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 6 is an enlarged view of a temperature-reducing material supply nozzle and a slab of FIG. 5;

FIG. 7 is a perspective view illustrating a modified example of equipment for manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 8 is a scanning electron microscopy (SEM) photograph to explain micro-structures of a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 9 is a SEM photograph to explain phase-transformation according to a surface cooling method of a slab for manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 10 is a SEM photograph to explain phase-transformation according to a surface cooling time of a slab for manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 11 illustrates characteristic variation according to a volume fraction of a ferrite structure of a surface portion of a hot-rolled steel sheet according to example embodiments of the inventive concepts;

FIG. 12 is a graph illustrating a tensile curve according to a coiling temperature of a hot-rolled steel sheet according to embodiments of the inventive concepts; and

FIGS. 13 to 16 illustrate application examples of a hot-rolled steel sheet according to embodiments of the inventive concepts.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The inventive concepts will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the inventive concepts are shown. The advantages and features of the inventive concepts and methods of achieving them will be apparent from the following exemplary embodiments that will be described in more detail with reference to the accompanying drawings. It should be noted, however, that the inventive concepts are not limited to the following exemplary embodiments, and may be implemented in various forms. Accordingly, the exemplary embodiments are provided only to disclose the inventive concepts and let those skilled in the art know the category of the inventive concepts. In the drawings, embodiments of the inventive concepts are not limited to the specific examples provided herein and are exaggerated for clarity.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular terms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it may be directly connected or coupled to the other element or intervening elements may be present.

Similarly, it will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present. In contrast, the term “directly” means that there are no intervening elements. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Additionally, the embodiment in the detailed description will be described with sectional views as ideal exemplary views of the inventive concepts. Accordingly, shapes of the exemplary views may be modified according to manufacturing techniques and/or allowable errors. Therefore, the embodiments of the inventive concepts are not limited to the specific shape illustrated in the exemplary views, but may include other shapes that may be created according to manufacturing processes. Areas exemplified in the drawings have general properties, and are used to illustrate specific shapes of elements. Thus, this should not be construed as limited to the scope of the inventive concepts.

It will be also understood that although the terms first, second, third etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element in some embodiments could be termed a second element in other embodiments without departing from the teachings of the present invention. Exemplary embodiments of aspects of the present inventive concepts explained and illustrated herein include their complementary counterparts. The same reference numerals or the same reference designators denote the same elements throughout the specification.

Moreover, exemplary embodiments are described herein with reference to cross-sectional illustrations and/or plane illustrations that are idealized exemplary illustrations. Accordingly, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, exemplary embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an etching region illustrated as a rectangle will, typically, have rounded or curved features. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments.

FIG. 1 is a flowchart illustrating a method of manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts.

Referring to FIG. 1, a slab may be hot-rolled (S110). The stab may be heated at 1250° C. for 2 hours and may be then hot-rolled. The slab having a thickness of 40 mm may be hot-rolled to form a pressed slab having a thickness of 3 mm. In some embodiments, the slab may be a board consisting of carbon (C) of 0.15 wt %, manganese (Mn) of 1.2 wt %, silicon (Si) of 0.3 wt %, niobium (Nb) of 0.03 wt %, boron (B) of 0.02 wt %, a residual iron (Fe), and other inevitable impurities. The hot-rolled slab may have an austenite structure.

A first phase transformation process may be performed to cool a surface of the hot-rolled slab (S120). During the first phase transformation process, the surface of the hot-rolled slab may be cooled such that the structure of a surface portion of the slab may be phase-transformed. The surface portion of the slab may include portions of the slab which are adjacent to a first surface and a second surface opposite to each other of the slab. The first surface and the second surface may be a top surface and a bottom surface of the slab, respectively.

For example, if the hot-rolled slab has the austenite structure before the first phase transformation process, the surface portion of the slab may have a ferrite structure by the first phase transformation process.

While the structure of the surface portion of the slab is phase-transformed by the first phase transformation process, a phase of a structure of a central portion of the slab may be maintained. For example, even though the first phase transformation process is performed, the central portion of the slab may have the austenite structure. In other words, the phase of the structure of the central portion of the slab may not be transformed during the first phase transformation process. The central portion of the slab may be a portion of the slab, which is disposed between the first and second surfaces of the slab.

In some embodiments, compressed air may be provided to the first surface and the second surface of the slab during the first phase transformation process. For example, air in the atmosphere may be compressed using a pump to form the compressed air. Alternatively, the compressed air may include a specific gas (e.g., an inert gas) compressed by a pump.

In other embodiments, a liquid temperature-reducing material may be jetted to the first surface and the second surface of the slab during the first phase transformation process. For example, the liquid temperature-reducing material may be one of water or liquid nitrogen.

The first and second surfaces of the slab may be cooled to a temperature of A3 or less by the provided compressed air or liquid temperature-reducing material, so that the structure of the surface portion of the slab may be phase-transformed. For example, the structure of the surface portion of the slab may be phase-transformed from the austenite structure to the ferrite structure.

The compressed air and the liquid temperature-reducing material may be supplied to the first surface and the second surface of the slab for a short time. Thus, the central portion of the slab may not be cooled during the first phase transformation process such that the phase of the structure of the central portion may be maintained.

After the structure of the surface portion of the slab is phase-transformed by the first phase transformation process, a second phase transformation process may be performed to phase-transform the structure of the central portion of the slab (S130). The central portion of the slab, which was not phase-transformed in the first phase transformation process as described above, may be phase-transformed by the second phase transformation process.

For example, if the central portion of slab, which was not phase-transformed during the first phase transformation process, has the austenite structure, the structure of the central portion of the slab may be phase-transformed to a martensite structure by the second phase transformation process. Alternatively, the central portion of the slab may have a high strength low alloy (HSLA) steel structure such as a dual phase (DP) steel structure, a transformation induced plasticity (TRIP) steel structure or a twinning induced plasticity (TWIP) steel structure after the second phase transformation process.

While the structure of the central portion of the slab is phase-transformed by the second phase transformation process, the phase of the structure of the surface portion of the slab may be maintained. For example, even though the second phase transformation process is performed, the surface portion of the slab may have the ferrite structure. In other words, the phase of the structure of the surface portion of the slab may not be transformed during the second phase transformation process.

An elongation of the structure, which is phase-transformed by the second phase transformation process, of the central portion of the slab may be lower than an elongation of the structure, which is phase-transformed by the first phase transformation process, of the surface portion of the slab. The strength of the phase-transformed structure by the second phase transformation process of the central portion of the slab may be higher than the strength of the phase-transformed structure by the first phase transformation process of the surface portion of the slab.

The slab applied with the first phase transformation process may be water-cooled during the second phase transformation process. For example, a temperature of the slab may decrease at a rate of 85° C. per a second, so that the slab may be water-cooled to a temperature in a range of 160° C. to 450° C. The second phase transformation process may be performed using an apparatus different from that of the first phase transformation process.

As described above, the surface portion and the central portion may have different structures from each other. For example, the surface portion may have the ferrite structure, and the central portion may have the martensite structure. On the other hand, a portion of the slab, which is disposed between the surface portion and the central portion, may have a dual phase structure. The portion having the dual phase structure of the slab is defined as a mixed portion.

The mixed portion of the slab may be phase-transformed by the first phase transformation process and may be phase-transformed by the second phase transformation process. In more detail, a portion of the mixed portion of the slab may be phase-transformed by the first phase transformation process, and the rest portion of the mixed portion of the slab may be phase-transformed by the second phase transformation process.

While a structure of the portion of the mixed portion is phase-transformed by the first phase transformation process, a structure of the rest portion of the mixed portion may not be phase-transformed. While the structure of the rest portion of the mixed portion is phase-transformed by the second phase transformation process, the portion phase-transformed in the first phase transformation process of the mixed portion may not be phase-transformed. Thus, the mixed portion may include a first phase transformed in the first phase transformation process and a second phase transformed in the second phase transformation process.

A volume fraction of the first phase may be higher than a volume fraction of the second phase in a region of the mixed portion, which is adjacent to the first and second surfaces. On the other hand, a volume fraction of the second phase may be higher than a volume fraction of the first phase in another region of the mixed portion, which is adjacent to the central portion.

For example, if the hot-rolled slab has the austenite structure before the first phase transformation process, the portion of the mixed portion of the slab may be phase-transformed from the austenite structure to the ferrite structure by the first phase transformation process. While the portion of the mixed portion is phase-transformed to the ferrite structure, the rest portion of the mixed portion may maintain its austenite structure. The rest portion of the mixed portion may be phase-transformed from the austenite structure to the martensite structure by the second phase transformation process. While the rest portion of the mixed portion is phase-transformed to the martensite structure, the previously phase-transformed portion of the mixed portion may maintain its ferrite structure. Thus, the mixed portion may have the dual phase structure of the ferrite and martensite structures.

The slab including the phase-transformed surface and central portions may be coiled after the second phase transformation process (S140). The slab may be water-cooled to the temperature in the range of about 160° C. to 450° C. in the second phase transformation process and may be then coiled. The coiled slab may be kept at a room temperature to temper. A tempering temperature of the coiled slab may be controlled according to a cooled degree of the slab in the second phase transformation process.

According to embodiments of the inventive concepts, the surface portion of the slab may have the structure (e.g., the ferrite structure) having a relatively high elongation by the first phase transformation process, and the central portion of the slab may have the structure (e.g., the martensite structure) having a relatively high strength by the second phase transformation process. Thus, the high elongation and high strength hot-rolled steel sheet may be manufactured.

Hereinafter, the hot-rolled steel sheet manufactured by the method of the aforementioned embodiment will be described with reference to FIGS. 2 and 3.

FIG. 2 is a perspective view illustrating a hot-rolled steel sheet according to example embodiments of the inventive concepts. FIG. 3 is a cross-sectional view taken along a line A-A′ of FIG. 2 to illustrate a hot-rolled steel sheet according to example embodiments of the inventive concepts.

Referring to FIGS. 2 and 3, a hot-rolled steel sheet 10 may include a first surface 10 a, a second surface 10 b, and a central portion. The first surface 10 a and the second surface 10 b may be opposite to each other. The central portion may be a portion of the hot-rolled steel sheet 10, which is disposed between the first surface 10 a and the second surface 10 b.

The hot-rolled steel sheet 10 may include a dual phase structure having a first phase and a second phase. The first phase of the hot-rolled steel sheet 10 may be generated in the first phase transformation process described with reference to FIG. 1, and the second phase of the hot-rolled steel sheet 10 may be generated in the second phase transformation process described with reference to FIG. 1.

The first phase may have the maximum volume fraction at the first surface 10 a and the second surface 10 b. The hot-rolled steel sheet 10 may have a first portion adjacent to the first surface 10 a and a second portion adjacent to the second surface 10 a. In some embodiments, the first and second portions of the hot-rolled steel sheet 10 may have only the first phase but may not have a structure of the second phase (hereinafter, referred to as ‘a second phase structure’). In other embodiments, the first and second portions may have a very small amount of the second phase structure. The volume fraction of the first phase may gradually decrease from the first surface 10 a to the central portion, and the volume fraction of the second phase may gradually increase from the first surface 10 a to the central portion.

The second phase may have the maximum volume fraction in the central portion. For example, the central portion of the hot-rolled steel sheet 10 may have the second phase but may not have a structure of the first phase (hereinafter, referred to as ‘a first phase structure’). In other embodiments, the central portion of the hot-rolled steel sheet 10 may have a very small amount of the first phase structure. The volume fraction of the second phase may gradually decrease and the volume fraction of the first phase may gradually increase from the central portion to the first surface 10 a and from the central portion to the second surface 10 b.

The first phase structure may have a higher elongation than the second phase structure. The second phase structure may have a higher strength than the first phase structure. For example, the first phase structure may be a ferrite structure, and the second phase structure may be a martensite structure or a high strength low alloy (HSLA) steel structure (e.g., a dual phase (DP) steel structure, a transformation induced plasticity (TRIP) steel structure, or a twinning induced plasticity (TWIP) steel structure).

A mixed portion, which is disposed between the first portion and the central portion or between the second portion and the central portion, may have both the first phase and the second phase. A volume fraction of the first phase of the mixed portion may gradually increase and a volume fraction of the second phase of the mixed portion may gradually decrease as a distance from each of the first and second surfaces 10 a and 10 b decreases. On the contrary, the volume fraction of the second phase of the mixed portion may gradually increase and the volume fraction of the first phase of the mixed portion may gradually decrease as a distance from the central portion decreases.

The first and second portions having only the first phase, the central portion having only the second phase, and the mixed portion having both the first and second phases will be described with reference to FIG. 4 in more detail.

FIG. 4 is an enlarged cross-sectional view taken along a line A-A′ of FIG. 2 to illustrate a hot-rolled steel sheet according to example embodiments of the inventive concepts.

Referring to FIG. 4, the hot-rolled steel sheet 10 may include a first portion 11, a first mixed portion 12, a central portion 13, a second mixed portion 14 and a second portion 15 which are sequentially stacked. The first portion 11, the first mixed portion 12, the central portion 13, the second mixed portion 14 and the second portion 15 may be formed in one body.

The first portion 11 and the second portion 15 may have only a first phase, and the central portion 13 may have only a second phase different from the first phase. Each of the first and second mixed portions 12 and 14 may have both the first phase and the second phase.

A structure of the first phase structure may have a higher elongation than a structure of the second phase. The second phase structure may have a higher strength than the first phase structure. For example, the first phase structure may be a ferrite structure, and the second phase structure may be a martensite structure or a high strength low alloy (HSLA) steel structure (e.g., a dual phase (DP) steel structure, a transformation induced plasticity (TRIP) steel structure, or a twinning induced plasticity (TWIP) steel structure).

In the first mixed portion 12, a volume fraction of the first phase may become progressively greater toward the first portion 11, and a volume fraction of the second phase may become progressively greater toward the central portion 13. Thus, the volume fraction of the first phase may be higher than the volume fraction of the second phase in a region of the first mixed portion 12, which is adjacent to the first portion 11. The volume fraction of the second phase may be higher than the volume fraction of the first phase in another region of the first mixed portion 12, which is adjacent to the central portion 13.

In the second mixed portion 14, a volume fraction of the first phase may become progressively greater toward the second portion 15, and a volume fraction of the second phase may become progressively greater toward the central portion 13. Thus, the volume fraction of the first phase may be higher than the volume fraction of the second phase in a region of the second mixed portion 14, which is adjacent to the second portion 15. The volume fraction of the second phase may be higher than the volume fraction of the first phase in another region of the second mixed portion 14, which is adjacent to the central portion 13.

Hereinafter, equipment for manufacturing the hot-rolled steel sheet of the aforementioned embodiment will be described with reference to FIGS. 5 to 7.

FIG. 5 is a perspective view illustrating equipment for manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts. FIG. 6 is an enlarged view of a temperature-reducing material supply nozzle and a slab of FIG. 5 to explain the temperature-reducing material supply nozzle included in the equipment.

Referring to FIGS. 5 and 6, the equipment for manufacturing the hot-rolled steel sheet may include a roller 210 hot-rolling a slab S, a transport table 220 transporting the hot-rolled slab S in a first direction, a first surface cooling part 230, a second surface cooling part 240, and a third cooling part 250.

The first surface cooling part 230 may be disposed over the transport table 220. The first surface cooling part 230 may cool a first surface of the slab S. The first surface may be a top surface of the slab S.

The second surface cooling part 240 may be disposed under the transport table 220. The first surface cooling part 230 and the second surface cooling part 240 may be spaced apart from each other with the transport table 220 and the slab S therebetween. The second surface cooling part 240 may cool a second surface of the slab S. The second surface may be a bottom surface of the slab S, which is supported by the transport table 220.

Each of the first and second surface cooling parts 230 and 240 may include a temperature-reducing material supply line 232 and a temperature-reducing material supply nozzle 234. The temperature-reducing material supply 232 may extend in a second direction perpendicular to the first direction in which the slab S is transported by the transport table 220. The temperature-reducing material supply nozzle 234 may be installed on the temperature-reducing material supply line 232. The temperature-reducing material supply nozzle 234 may supply a temperature-reducing material supplied through the temperature-reducing material supply line 232 to the first surface or the second surface of the slab S. The temperature-reducing material supply nozzle 234 may be provided in plural to the temperature-reducing material supply line 232, and the plurality of temperature-reducing material supply nozzles 234 may be spaced apart from each other in the first direction. The shapes of the first and second surface cooling parts 230 and 240 may be symmetric.

The temperature-reducing material provided through the temperature-reducing material supply nozzle 234 to the first or second surface of the slab S may be, for example, compressed air or a liquid temperature-reducing material. If the temperature-reducing material is the liquid temperature-reducing material, it may be jetted or sprayed from the temperature-reducing material supply nozzle 234 toward the first surface or the second surface.

The temperature-reducing material supply nozzles 234 of the first and second surface cooling parts 230 and 240 may supply the temperature-reducing material to the first surface and the second surface of the slab S in an opposite direction to the first direction in which the slab S is transported. Thus, the temperature-reducing material supplied from the temperature-reducing material supply nozzles 234 may become in contact with the first and second surfaces of the slab S and may be then moved in the opposite direction to the first direction. As a result, the first and second surfaces of the slab S may be cooled in a moment, and the temperature-reducing material may not be supplied again to the surface of the slab S already supplied with the temperature-reducing material. Thus, a central portion of the slab S may not be cooled but a first portion of the slab S adjacent to the first surface and a second portion of the slab S adjacent to the second surface may be cooled.

The first portion and the second portion of the slab S may be selectively cooled by the first and second surface cooling parts 230 and 240, thereby performing the first phase transformation process described with reference to FIG. 1. In more detail, the first and second surfaces of the slab S may be cooled by the first and second surface cooling parts 230 and 240, so that the first and second portions adjacent to the first and second surfaces of the slab S may be phase-transformed. While the slab S is cooled by the first and second surface cooling parts 230 and 240, the central portion of the slab S may not be cooled such that a phase structure of the central portion of the slab S may not be transformed.

The third cooling part 250 may supply large amounts of water to the slab S to water-cool the slab S. The second phase transformation process described with reference to FIG. 1 may be performed by the third cooling part 250. In more detail, the central portion of the slab S may be phase-transformed by the water supplied from the third cooling part 250. At this time, the phase structures of the first and second portions adjacent to the first and second surfaces may not be transformed.

In the aforementioned embodiment and FIG. 5, each of the first and second surface cooling parts 230 and 240 may supply the temperature-reducing material through one temperature-reducing material supply line 232 and the temperature-reducing material supply nozzle connected thereto. In other embodiments, each of the first and second surface cooling parts 230 and 240 may include a plurality of temperature-reducing material supply lines 232. This will be described with reference to FIG. 7.

FIG. 7 is a perspective view illustrating a modified example of equipment for manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts.

Referring to FIG. 7, the equipment according to the present modified example may include the roller 210, the transport table 220 transporting the hot-rolled slab S in the first direction, a first surface cooling part 230, a second surface cooling part 240, and the third cooling part 250.

Unlike the equipment described with reference to FIG. 5, each of the first and second cooling parts 230 and 240 may include a plurality of temperature-reducing material supply lines 232 and 233. The plurality of temperature-reducing material supply lines 232 and 233 may be arranged and spaced apart from each other in the first direction.

Physical characteristics of the hot-rolled steel sheet manufactured using the equipment by the method according to the aforementioned embodiments will be described hereinafter.

FIG. 8 is a scanning electron microscopy (SEM) photograph to explain micro-structures of a hot-rolled steel sheet according to example embodiments of the inventive concepts.

Referring to FIG. 8, a slab was prepared. The slab was formed to consist of carbon (C) of 0.15 wt %, manganese (Mn) of 1.2 wt %, silicon (Si) of 0.3 wt %, niobium (Nb) of 0.03 wt %, boron (B) of 0.02 wt %, a residual iron (Fe), and other inevitable impurities. The slab has a thickness of 40 mm and an austenite structure. Compressed air was provided to a hot-rolled slab to perform the first phase transformation process described with reference to FIG. 1. Thus, a surface portion of the slab was phase-transformed from the austenite structure to a ferrite structure by the first phase transformation process. Thereafter, the slab was water-cooled to perform the second phase transformation process described with reference to FIG. 1. Thus, a central portion of the slab was phase-transformed from the austenite structure to a martensite structure.

As illustrated in FIG. 8, the surface portion of the slab supplied with the compressed air, had the ferrite structure having a relatively high elongation, and the central portion of the slab had the martensite structure having a relative high strength.

Additionally, a mixed portion between the surface portion and the central portion had both the ferrite structure and the martensite structure. In particular, a volume fraction of the ferrite structure of the mixed portion became progressively greater increased toward the surface portion, and a volume fraction of the martensite structure of the mixed portion became progressively greater toward the central portion.

The following table 1 shows a result of a tensile experiment of the hot-rolled steel sheet according to the embodiments of the inventive concepts. The tensile experiments were performed with varying a coiling temperature of the hot-rolled steel sheet described with reference to FIG. 8. In more detail, each of hot-rolled steel sheets of embodiments 1 to 4 in the following table 1 included the surface portion having the ferrite structure generated by the first phase transformation process. The generated ferrite structure had a thickness of 500 μm. For the tensile experiment, the hot-rolled steel sheets of the embodiments 1 to 4 were coiled at 450° C., 350° C., 270° C., and 160° C., respectively.

The ferrite structures generated by the first phase transformation process were removed from the hot-rolled steel sheets of the embodiments 1 to 4 to form hot-rolled steel sheets of comparison examples 1 to 4 in the following table 1. In other words, the hot-rolled steel sheets of the comparison examples 1 to 4 had only the martensite structures. The tensile experiment was also to the hot-rolled steel sheets of the comparison examples 1 to 4 at different coiling temperatures from each other.

TABLE 1 Coiling temperature YS TS E1 TS × E1 Embodiment 1 450° C. 971 1008 14 14112 Embodiment 2 350° C. 1063 1161 12.5 14513 Embodiment 3 270° C. 998 1251 12 15012 Embodiment 4 160° C. 885 1233 14 17262 Comparison example 1 450° C. 1116 1220 4.7 5734 Comparison example 2 350° C. 1228 1347 4.5 6061.5 Comparison example 3 270° C. 1148 1367 4.4 6014.8 Comparison example 4 160° C. 1038 1399 5.3 7414.7

Referring to the table 1, the comparison examples 1 to 4 not having the ferrite structures generated from the first phase transformation process had high tensile strengths of about 1.2 Gpa or more and low elongations of about 5%. Additionally, the comparison examples 1 to 4 had low values of strength X ductility corresponding to an index of energy absorbing ability. These characteristics of the comparison examples 1 to 4 correspond to mechanical properties of a general martensite steel. The strength X ductility of the comparison examples 1 to 4 is low such that the availability of the comparison examples 1 to 4 is also low.

On the contrary, the embodiments 1 to 4 having the ferrite structure of the surface portion generated by the first phase transformation process had a low tensile strength of about 200 Mpa. Additionally, elongations of the embodiments 1 to 4 increased by about 7%. Moreover, values of strength X ductility of the embodiments 1 to 4 were more than double the values of the strength X ductility of the comparison examples 1 to 4. In other words, the hot-rolled steel sheet according to the embodiments of the inventive concepts includes the surface portion having the ferrite structure of the relatively high elongation and the central portion having the martensite structure of the relatively high strength, so that physical characteristics of the hot-rolled steel sheet are excellent.

The following table 2 shows a result of a bending experiment performed on the hot-rolled steel sheets of the embodiments 1 to 4 described with reference to the table 1. The following table 3 shows a bending experiment result of a martensite steel which has a tensile strength of 1180 Mpa and does not have the ferrite structure in its surface portion. The bending experiments were performed under conditions of radiuses of curvature of 1 mm R, 2 mm R, 3 mm R, and 5 mm R. In the following tables 2 and 3, “0” denotes that a crack does not occur, “Δ” denotes that a fine crack occurs, and “X” denotes that a crack occurs.

TABLE 2 1 mm R 2 mm R 3 mm R 5 mm R Embodiment 1 Δ Δ ◯ ◯ Embodiment 2 Δ Δ ◯ ◯ Embodiment 3 ◯ ◯ ◯ ◯ Embodiment 4 ◯ ◯ ◯ ◯

TABLE 3 1180 1 mm R 2 mm R 3 mm R 5 mm R Grade t Top Tail Top Tail Top Tail Top Tail MS A 2.0 X X X X X X X X MS B 2.0 X X X X ◯ X ◯ X MS C 4.0 X X X X X X X X MS D 4.0 ◯ X ◯ X X X ◯ X

Referring to the tables 2 and 3, fine cracks occurred when the bending experiments were performed to the hot-rolled steel sheets, which were coiled at 450° C. and 350° C. according to of the embodiments 1 and 2, under the conditions of the radiuses of curvature of 1 mm R and 2 mm R. However, the crack didn't occur when the bending experiments were performed to the hot-rolled steel sheets of the first and second embodiments under the conditions of the radiuses of curvature of 3 mm R and 5 mm R. Additionally, the crack didn't occur when the bending experiments were performed to the hot-rolled steel sheets which were coiled at 270° C. and 160° C. according to of the embodiments 3 and 4. Thus, a bending characteristic of the hot-rolled steel sheet having the ferrite structure in its surface portion is markedly improved as compared with that of the hot-rolled steel sheet not having the ferrite structure.

FIG. 9 is a SEM photograph to explain phase-transformation according to a surface cooling method of a slab for manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts, and FIG. 10 is a SEM photograph to explain phase-transformation according to a surface cooling time of a slab for manufacturing a hot-rolled steel sheet according to example embodiments of the inventive concepts.

Referring to FIG. 9, the slab of FIG. 8 was hot-rolled and the hot-rolled was then air-cooled for 3 seconds in the atmosphere to perform the first phase transformation process described with reference to FIG. 1. Thus, a sample (a) of FIG. 9 was obtained. On the other hand, the slab of FIG. 8 was hot-rolled and the compressed air was supplied using an air gun to the hot-rolled slab for 3 seconds to perform the first phase transformation process described with reference to FIG. 1. Thus, a sample (b) of FIG. 9 was obtained.

The ferrite structure was formed to have a thickness of about 20 μm when the hot-rolled slab was air-cooled for 3 seconds in the atmosphere. The ferrite structure was formed to have a thickness of about 100 μm when the hot-rolled slab was cooled using the compressed air for 3 seconds.

Referring to FIG. 10, a cooling time of the surface portion of the slab of FIG. 8 was controlled using an air gun. A thickness of the ferrite structure progressively increased as the cooling time (i.e., a use time of the air gun) increased. In other words, the cooling method and the cooling time may be controlled to control a thickness of a phase-transformed structure in the surface portion of the slab.

FIG. 11 shows characteristic variation according to a volume fraction of a ferrite structure of a surface portion of a hot-rolled steel sheet according to example embodiments of the inventive concepts.

Referring to FIG. 11, a formation volume fraction of the ferrite structure on the surface portion of the slab described in FIG. 8 was varied. A first sample (a) of FIG. 11 denotes that a ferrite structure having a relatively low volume fraction was formed from the surface of the slab to a depth of about 160 μm. A second sample (b) of FIG. 11 denotes that a ferrite structure having a relatively high volume fraction was formed from the surface of the slab to a depth of about 280 μm. A third sample (c) of FIG. 11 denotes that a ferrite structure having a relatively low volume fraction was formed from the surface of the slab to a depth of about 500 μm.

A bending experiment was performed to the first to third samples (a), (b) and (c). As a result, a bending characteristic of the second sample (b) including the ferrite structure having the relatively high volume fraction was better than those of the first sample (a) including the ferrite structure having the relatively low volume fraction and the thin thickness and the third sample (c) including the ferrite structure having the relatively low volume fraction and the thick thickness. In other words, in the first phase transformation process described with reference to FIG. 1, the influence of the volume fraction of the phase-transformed ferrite structure on the bending characteristic is more than the influence of the thickness of the phase-transformed ferrite structure on the bending characteristic.

FIG. 12 is a graph illustrating a tensile curve according to a coiling temperature of a hot-rolled steel sheet according to embodiments of the inventive concepts.

Referring to FIG. 12, the first and second phase transformation processes of FIG. 1 were sequentially performed on the slab described with reference to FIG. 8 to form the hot-rolled steel sheet. The hot-rolled steel sheets were coiled with varying a temperature. Tensile strengths of the hot-rolled steel sheets coiled at different temperatures were measured. As described with reference to the table 1, the hot-rolled steel sheets of the embodiments 1 to 4 were coiled at 450° C., 350° C., 270° C., and 160° C., respectively.

As shown in FIG. 12, the tensile strength increased as the coiling temperature increased from 160° C. to 270° C. When the coiling temperature was equal to or greater than 270° C., the tensile strength decreased as the coiling temperature increased. On the contrary, the elongation decreased as the coiling temperature increased from 160° C. to 270° C. When the coiling temperature was equal to or greater than 270° C., the elongation increased as the coiling temperature increased.

FIGS. 13 to 16 show application examples of a hot-rolled steel sheet according to embodiments of the inventive concepts.

Referring to FIGS. 13 to 16, the hot-rolled steel sheet according to embodiments of the inventive concepts may be used in order to form a car part. The hot-rolled steel sheet according to embodiments of the inventive concepts may be formed into a B-pillar 310, a side impact beam 320 or a wheel 330 of a car. In other embodiments, the hot-rolled steel sheet according to embodiments of the inventive concepts may also be formed into other various car parts such as a bumper reinforcing member. As described above, the hot-rolled steel sheet according to embodiments of the inventive concepts has the high strength and the high elongation, so that it may be highly useful in the car parts.

The hot-rolled steel sheet according to embodiments of the inventive concepts may be used as the car parts as described with reference to FIGS. 13 to 16. However, the inventive concepts are not limited thereto. The hot-rolled steel sheet according to embodiments of the inventive concepts may be widely used in various industrial fields requiring high strength and high elongation characteristics.

According to embodiments of the inventive concepts, after the structure of the surface portion of the slab is phase-transformed, the central portion of the slab may be phase-transformed to form the hot-rolled steel sheet. The surface portion of the hot-rolled steel sheet includes the structure having the relatively high elongation, and the central portion of the hot-rolled steel sheet includes the structure having the relatively high strength. As a result, the hot-rolled steel sheet having the high elongation and the high strength may be realized.

While the inventive concepts have been described with reference to example embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirits and scopes of the inventive concepts. Therefore, it should be understood that the above embodiments are not limiting, but illustrative. Thus, the scopes of the inventive concepts are to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing description. 

What is claimed is:
 1. A method of manufacturing a hot-rolled steel sheet, the method comprising: hot-rolling a slab; performing a first phase transformation process cooling a surface of the hot-rolled slab to phase-transform a structure of a surface portion of the hot-rolled slab; performing a second phase transformation process to phase-transform a structure of a central portion of the hot-rolled slab after the first phase transformation process; and coiling the slab including the phase-transformed surface portion and the phase-transformed central portion.
 2. The method of claim 1, wherein a phase of the structure of the central portion of the hot-rolled slab is maintained during the first phase transformation process.
 3. The method of claim 2, wherein a phase of the structure of the surface portion of the hot-rolled slab is maintained, during the second phase transformation process.
 4. The method of claim 1, wherein performing the first phase transformation process comprises: providing compressed air to the surface portion of the hot-rolled slab.
 5. The method of claim 1, wherein performing the first phase transformation process comprises: jetting a liquid temperature-reducing material to the surface portion of the hot-rolled slab.
 6. The method of claim 1, wherein the hot-rolled slab has an austenite structure, wherein the surface portion of the hot-rolled slab has a ferrite structure by the first phase transformation process, and the central portion of the hot-rolled slab has the austenite structure after the first phase transformation process and before the second phase transformation process, and wherein the central portion of the hot-rolled slab has a structure having a higher strength than the ferrite structure by the second phase transformation process, and the surface portion of the hot-rolled slab has the ferrite structure after the second phase transformation process.
 7. The method of claim 1, wherein an elongation of the surface portion phase-transformed by the first phase transformation process is higher than an elongation of the central portion phase-transformed by the second phase transformation process, and wherein a strength of the central portion phase-transformed by the second phase transformation process is higher than a strength of the surface portion phase-transformed by the first phase transformation process.
 8. The method of claim 1, wherein performing the second phase transformation process comprises: water-cooling the hot-roiled slab.
 9. The method of claim 1, further comprising: tempering the coded slab.
 10. The method of claim 1, wherein the first phase transformation process and the second phase transformation process are performed using different apparatuses from each other.
 11. A hot-rolled steel sheet comprising: a first surface; a second surface opposite to the first surface; and a central portion between the first and second surfaces, wherein a first phase has a maximum volume fraction and a second phase has a minimum volume fraction at the first surface and the second surface, and wherein the first phase has a minimum volume fraction and the second phase has a maximum volume fraction in the central portion.
 12. The hot-rolled steel sheet of claim 11, wherein a structure of the first phase is a ferrite structure, and wherein a structure of the second phase has a higher strength than the ferrite structure.
 13. The hot-rolled steel sheet of claim 11, wherein a structure of the first phase has a higher elongation than a structure of the second phase, and wherein the structure of the second phase has a higher strength than the structure of the first phase.
 14. The hot-rolled steel sheet of claim 11, wherein first and second portions respectively adjacent to the first and second surfaces have only the first phase, and wherein the central portion has only the second phase.
 15. The hot-rolled steel sheet of claim 11, wherein the volume fraction of the first phase progressively decreases from the first surface to the central portion, and the volume fraction of the second phase progressively increases from the first surface to the central portion, and wherein the volume fraction of the first phase progressively increases from the central portion to the second surface, and the volume fraction of the second phase progressively decreases from the central portion to the second surface.
 16. Equipment for manufacturing a hot-rolled steel sheet, the equipment comprising: a roller hot-rolling a slab; a transport table transporting the hot-rolled slab; a first surface cooling part disposed over the transport table, the first surface cooling part cooling a first surface of the hot-rolled slab; a second surface cooling part spaced apart from the first surface cooling part with the transport table therebetween, the second surface cooling part cooling a second surface of the hot-rolled slab opposite to the first surface; and a third cooling part water-cooling the slab cooled by the first and second surface cooling parts.
 17. The equipment of claim 16, wherein the transport table transports the slab in a first direction, and wherein each of the first and second surface cooling parts comprises: a temperature-reducing material supply line extending in a second direction perpendicular to the first direction; and a temperature-reducing material supply nozzle supplying a temperature-reducing material supplied from the temperature-reducing material supply line to the first surface or the second surface of the hot-rolled slab.
 18. The equipment of claim 17, wherein the temperature-reducing material supply nozzle supplies the temperature-reducing material to the first surface or the second surface of the slab in an opposite direction to the first direction.
 19. The equipment of claim 17, wherein the temperature-reducing material supply nozzle is disposed to be oblique with respect to the first surface or the second surface of the slab.
 20. The equipment of claim 16, wherein a first portion of the slab adjacent to the first surface and a second portion of the slab adjacent to the second surface are phase-transformed by the first and second surface cooling, parts, and wherein a phased of a central portion of the slab between the first and second portions is maintained when the first and second portions are phase-transformed. 